Method of preheating the charge in shaft furnaces



uw 18,1928. i 1,695,953V

A. L. FEILD METHOD OF PREHEATING THE CHARGE IN SHA-FT, FURNACES FiledApril 25, 1924 SHO: nur.

Patented Dec. 18, 1928.

UNITED STATES PATENT OFFICE.

ALEXANDER L. EEILD, or JACxsON HEIGHTS, NEW YORK, AssTGNon To THE EINDEf Am PRODUCTS COMPANY, A CORPORATION or OHTO.

METHOD OF IEREEIEIATIIIG' TEE CHARGE IN SHAFT FURNACES.

Application led Apr125, 1924. Serial No. 709,012.

This invention relates to metallurgical operations conducted in shaftfurnaces, or the like, and comprises an improved method of preheatingvthe charge in the furnace.

The invention is particularly applicable in the blast smelting .of ironores.. In the following description reference will therefore be madeprimarily to this use, but it is to'be understood that the improvedmethod is advantageous in other metallurgical processes.

In standard blast furnace practice the charge fed downwardly through thestack receives heat from two principal' sources. These are the heat ofcombustion in the tuyre zone of the furnace and the heat of the airpassing to the furnace from thev preheating stoves. Theflatter source ofheat is a very important factor in the proper working of the furnace.When air is supplied from the stoves at 600 C. or above, according tousual practice, its heat content may amount to 20% or more of the heatproduced by combustion in the furnace. About 75% of the heat of the airblast is carried b the nitrogen which it contains. The weig t o-fnitrogen passing through the furnace is approximately three times theweight of the iron produced. Nevertheless, the importance of the heatingeffect 'of this gas does 3G not appear 'to have been .accuratelyevaluated heretofore.

I have discovered that any material decrease in the nitrogen content ofthe air will result in a serious vdepletion of the thermal unitsavailable for preheating the charge.

A failure to recognize this fact has been responsible for theditliculties encountered in the use of oxygen or oxygenated air in blastfurnace practice. v

Such proposals have been made at various times during the last twentyyears. So far as I am aware, however, no continuous in-y dustrial, use,has been made of them. It is true that an oxygenated blast produces ahigher reaction temperature in the smelting zone than is obtainable withan ordinary air blast, but the advantages of this higher temperature arelargely discounted by the fact that the charge' is fed into the smeltingzone without sufficient preheating. l* This defect is a necessary resultof operating with an inadequate volume of hot gases ascending throughVthe charge, which requires a very large amount of heat to bring it tothe proper temperature, Heat supplied from combustion of coke or othersource should raise the temperature of the burden (iron ore and flux) toabout 1500o C., or

above, before it enters the smelting region,

if the furnace vis to operate yto the best advantage. The burdenundergoes no strongly exothermic chemical reaction. The ore mustordinarily be dried, and the limestone flux calcined, in the shaft. Thecooling effect of the descending charge is augmented by thewater-cooling of `the furnace in the tuyre zone.

A higher combustion temperature does not itself compensate for theseheat losses, or insure proper preheating. By lowering the burden ratio(pounds ofv ore and flux to pounds of coke) the action may be improvedsomewhat, but such procedure is uneconomical. No substantial advantageis derived from an increased rapidity of descent of the charge, causedby quicker smelting. Any tendency in this direction is largely impededby the greater heat absorbing capacity of the charge when fed at a morerapid rate.'

In accordance with the present invention, difficulties arising frominsufficient preheating are avoided by passing into the furnace, at apoint or points above the smelting Zone,

a hot gas'incapable of producing any substantial smelting of the charge.

. Nitrogen, or nitrogen in admixture with some oxygen, is preferred, butother suitable gases or Vgaseous mixtures may be used. Considerable heatmay be safelyderived by the combustion of fuel in the charge by oxygenof the preheatinggas. The carbon monoXid so generated is advantageous,as it assists in the reduction of the ore. In all cases, however, theoxygen content of that gasshould be materially less than thatl ofatmospheric air. An' air blast at a point above. thel ordinary tuyrelevel is to be avoided, as coke would be needlessly consumed in largequantities and the resulting fusion of the charge above the lower tuyreswould have serious disadvantages. By using a gas low in oxygen I am ableto avoid these difficulties.`

The preheating gas as it enters the furnace may contain some carbonmonoxid or other reducing gases. However, I do not use top gases fromvthe stack, nor other gases containing a' high Vroportion of the oXids ofcarbon, for pre eating the charge. Such gases are sometlmes recirculatedfor the pur- .furnace by the present process. The

vIn the accompanying drawings, illustra tive of the invention,v

Fig. 1 is a diagrammatic partial'vertical section through a blastfurnace provided with preheating means and Fig. 2 is a diagram of a.blast furnace plant using oxygenated air and a preheating gas rich innitrogen.

Referring to Fig. 1, numeral 1 denotes the shaft of a furnace of usualtype, having a hearth 2, cinder notch 3 and metal tap hole 4. Tuyres 5are installed in the customary manner.

To carry'out the purposes of this invfention,'one or more tuyres or setsof tuyres are provided, preferably above those already referred to. Asillustrative, four tuyres 6 (of which three only are shown) are placedsymmetrically somewhat above the tuyeres 5. This .location is deemedespecially advantageous, as substantially the maximum preheating effectof the gases is obtained. However, the tuyres 6 may be otherwisearranged, if desired, both with respect to their level and'to theircircumferential position.

The preheating gas (nitrogen, vnitrogenoxygen mixture, or the like) isheated to the proper temperature in stoves'and forced into the shaftunder suitable pressure, say 12 to 15 lbs. per sg. in. The volume ofpre-4 heated gas required may be readily determined theoretically forgiven conditions by means of published data familiar to those skilled inthe art.

The smelting is preferably effected by heated air blown in throughtuyres 5. However, .the present inventlon is e ually applicable to coldblast smelting. t has special advantages in connection w1 th the use ofan oxygenated blast, as hereinafter -described in connection withFig. 2.

In that figure, reference numerals 5 and 6 denote as before the tuyresrespectively serving to introduce the smelting and preheating gases intothe furnaces. Stoves 7, 7 are respectively connected by pipe line 8 withtuyres 5 and by pipe line 9 with tuyres 6. While the stoves areindicated as units, it will be' understood that subdivisionsy areprovided, as well understood in the art, to permit the heating of onesection while another is receivin 4the air passingto the furnace. Aplant or preparing from air a vfraction enriched in oxygen is indicatedby ,numeral 10. 'Ihis fraction is passed through pipe 11 to stoves-7 andthence to tuyres 5. The residual fraction of the air, containing a highproportion of nitrogen, is passed through pipe 12 to stoves 7 and-thence to tuyres 6. A blowing engine forces the air fractions to thestoves. I A preheating gas is led to the stoves from the blast furnacestack as indicated or from some other suitable source.

Important improvements in the operation of the furnace are produced bythis distributed introduction of the air. The tempervature inthe hearthand bosh regions is very high, owing to the augmented oxygenconcentration of the smelting gas. Hence the rate of smelting, andconsequently the rate ofl feed of the charge, is much increased. Thecharge is properly preheated by the non-smelting gas, so thatthesmelting takes place without' the handicap of a' cooling charge. Theoutput of the furnace per' unit weight of fuel consumed is materially.raised by the new process.

There are also numerous incidental advani tages inherent in the process.For example, `it is practicable to use low grade ores or flux which cannot be satisfactorily worked known. I generally prefer to have at least30% of oxygen by weight in the smelting gas, but more or less may beused under particular conditions. If this gas is prepared from air, theresidual gas will contain about 18% of oxygen, and is excellently suitedfor the preheatin gas. The advantages of the invention, as gdescribedabove, are in no way dependent upon the fractionation of air to whichreference has been particularly made. Oxygenated and nitrogenated gas orthe like may be obtained from any suitable source. The preheating gas,as well as the smelting gas, may be dried, if desired. While I regardthe sensible heatl of the gas blown in at the upper tuyre level as theprimary source of heat, more or less heat may be generated various otherconnections, with or without modification, and is therefore definedbroadly in the appended claims:

I claim: 1. Process of preheating the charge in shaft furnaces,comprising passing a hot,

non-smelting gasl ycomprising an oxygenfl nitrogen mixture in whlch theratio of oxy- -gen to nitrogen is lessthan that in atmosi pheric airinto the furnace above the smelting zone. f

2. Process of preheating the ore-iluX-carbon charge in a blast furnace,comprising lpassing into the furnace another gas, comprising anoxygen-nitrogen mixture contalning less oxygen than atmospheric air, butcarrying sufficient heat to preheat the charge at least in part.

4. Process of `operating shaft furnaces,

comprising passing into the charged furnace a gas adapted to produce thedesired reduction reaction in the charge, preheating another gas,comprlsing an oxygen-nitrogen mixture contaimng less oxygen thanatmospheric air, and passing it in heat-interchanging relation to thecharge.

5. Process of operating shaft furnaces, comprising passing the furnacecharge downwardly in Contact with an ascending current-of a hot gasintroduced into the furnace at a pointl above the site of the mainreaction therein, said gas being substantially incapable of producingthe desired reaction in the charge and. comprising an oxygennitrogenmlxture containing less oxygen than atmospheric air, and separatelyintroducing'adjacent to the base of the furnace a gas adapted to effectsuch reaction.

6. Process of operating blast furnaces for the smelting of ores,comprising passing into the charged'furnace heated air divided into aplurality of fractions with different contents of oxygen, the portionhigh in oxygen passing to the smeltlng zone, and the portion low inoxygen ascending through the charge to preheat the same.

7. Process of operating blast furnaces for the smelting of iron ores,comprising passing into the charged smelting zone a gas containing aproportion of oxygen higher than that in atmospheric air, and passingthrough the bulk of the charge a hot gas containing a proportion ofoxygen lower l than that in. atmospheric air and ha :ing low reducingpower.

8. Process of operating blast furnaces for the smelting of iron ores,comprising pre'- paring from air a gas containlng a materially increasedproportion of oxygen, passing said gas into the charged smelting zone ofthe furnace, prehcating the residue of the air treated, and passingl itthrough the charge, whereby the charge is heated to the propertemperature before entering the smelting zone.'

In testimony whereof, I ax my signature.

ALEXANDER L. FEILD.y

